1. Field of the Invention
The present invention relates to communications; more specifically, wireless communications.
2. Description of the Related Art
In a TDMA (Time Division Multiple Access) system, communication channels between a base station and a mobile communication device are defined in terms of a frequency and time slot. Each communication channel is assigned to a frequency and one of multiple time slots transmitted on a given frequency. In the case of systems adhering to TIA (Telecommunications Industry Association) Standard IS-136.2, three time slots are associated with each frequency. As a result, three channels are assigned to each frequency.
FIG. 1 illustrates an arrangement of time slots as defined by Standard IS-136.2. A sequence of frames is transmitted where each frame contains six time slots. Each time slot contains the data associated with a communication channel. The half-frames contain 60 milliseconds of information where each time slot contains 20 milliseconds of information. Each time slot contains 324 bits. As a result, for a given communication channel, 324 bits of information are transmitted each half-frame.
FIG. 2 illustrates the different fields associated with the 324 bits from one of the slots of FIG. 1. The fields are defined by Standard IS-136.2. Field 40 is labeled G and is used as a guard field between slots and contains 6 bits. Field 42 is a ramp field having 6 bits which are used to provide time for a mobile transmitter to reach full operating power after being off for the previous slots. DATA field 44 is 16 bits long and is used to transmit data. SYNCH field 46 is 28 bits long and is used to transmit a synchronization pattern. DATA field 48 is 122 bits long and is used to transmit data. SACCH field 50 is 12 bits long and is used to transmit the SACCH message (Slow Associated Control Channel). This field is used to exchange signaling messages between the base station and mobile device. These signaling messages include information regarding the quality of the channel. CDVCC field 52 is 12 bits long and is used to transmit the CDVCC message (Coded Digital Verification Color Code). This channel is used to indicate that the base station and mobile device are exchanging proper data. DATA field 54 is 122 bits long and is used to transmit data. The data fields are typically used to carry information such as speech data. The Standard also permits replacing data fields 48 and 54 with a FACCH field (Fast Associated Control Channel) message. This message is used to exchange information between the mobile and base station in situations such as handoffs. It should be noted that during this time the voice data is blanked so that the signaling message may be passed between the mobile and base station.
Communications between a base station and a mobile communication device include messages containing data corresponding to speech. In many conversations, it is typical for moments of silence to occur. As a result, messages carried between the base station and mobile device are carrying data representative of silence. This is wasteful because mobile unit battery power is being used to transmit information representative of silence, and in addition, other channels are receiving interference from a channel that is only transmitting data indicative of silence.
One suggested solution to avoid wasting transmissions by sending data representative of silence or absence of voice is to stop transmitting in periods of silence except for transmissions containing information relating to channel quality. In a one second period of relative silence, only three transmissions rather than 50 transmissions would be sent. The three transmissions are in the 324 bit form of FIG. 2 and contain the channel quality data in DATA fields 44, 48, and 54. This solution offers the advantage of reducing co-channel interference and reducing battery drain at the mobile; however, it creates a problem for the party receiving the message. The party receiving the message will simply hear silence as opposed to background noise when no transmissions are made. This may make the users feel as if the call has been dropped when in reality it is simply a silence transmission.
A second suggested solution addresses this problem. Once again, a period of silence is detected using a voice activity detector, and transmission stops except for five transmissions per second. The five transmissions are in the 324 bit form of FIG. 2. Three of the five transmissions are used to transmit channel quality information in DATA fields 44, 48, and 54. The remaining two transmissions are used to transmit comfort noise information in same DATA fields. Comfort noise information is information representative of background noise that is transmitted to a receiver. The receiver uses the comfort noise information to generate background noise that a user may hear. As a result, in periods of silence a user still can hear background noise and therefore be assured that the communication channel has not been interrupted.
Both of these solutions suffer from the same problem. They both make it difficult for a base station to monitor channel quality when choosing an available channel for assignment to a new call. It should be noted that during a period of silence, the transmitter at the mobile is turned off for a large majority of the time (45 out of 50 possible transmission times). When the base station makes an interference measurement, it may get a false low interference reading based on a mobile that is temporarily not transmitting. As a result, the base station may assign a low quality channel to a new call.